Humber Admissions Practice Test

What is the key feature of codominance compared to incomplete dominance?

1. Both alleles are expressed partially in codominance

2. Both alleles are fully expressed in codominance

3. Only the dominant allele is shown in codominance

4. Codominance does not occur in plants

The correct answer is: Both alleles are fully expressed in codominance

In codominance, both alleles in a heterozygous genotype are fully expressed, resulting in a phenotype that distinctly reflects both traits. This means that when an individual carries two different alleles for a trait, such as in blood types, both alleles contribute to the phenotype without blending; instead, they coexist. For instance, in the case of AB blood type, both A and B antigens are fully present on the red blood cells. In contrast, incomplete dominance, where neither allele is completely dominant, results in a blending of traits. For example, if one allele is red and the other is white, the offspring may exhibit a pink phenotype, which is a mix rather than a full expression of either parent characteristic. The other options present inaccuracies. While one might think that codominance involves partial expression, it actually showcases full phenotypic representation of both traits. The notion that only the dominant allele is expressed contradicts the very principles of codominance, where both traits are visible. Lastly, saying that codominance does not occur in plants is misleading since it is a genetic phenomenon that can manifest across various organisms, including plants.